Showing papers by "Bulgarian Academy of Sciences published in 1974"

Some features of different motor units in human biceps brachii.

Abstract: Use has been made of the method of selective leading-off of impulses from individual motor units at high isometric muscle tension, the maximum one including. Two types of motor units have been distinguished in m. biceps brachii in man on the basis of the dependence between the firing frequency and the level of the muscle tension. Characteristic of the first (tonic) type is the rise in the frequency at lower tensions and constant frequency at higher tensions. The second (phasic) type shows an approximately linear rise of the frequency with increase of tension. The tonic motor units are smaller in size, with lower threshold, they are less fatiquable and their transient pattern of firing depends on the rate of increase of tension. They contribute less to the growth of the muscle tension. The phasic motor units are bigger in size with higher threshold, more fatiguable, their transient pattern of firing does not depend on the rate of increase of tension. They contribute essentially to the growth of the muscle tension.

Effect of surfactant concentration on the critical thicknesses of liquid films

Abstract: It is shown that an increase in surfactant concentration at first causes the critical thicknesses of rupture on microscopic films to decrease but gradually a nearly constant value is reached. With films of aqueous solutions of fatty acids (valeric, caproic, caprylic and capric acids), this dependence correlates well with the effect of the surfactant concentration on the damping of capillary waves. With surfactants of the detergent type (OPE-7, OPE-20) in addition to the change of the critical thickness, the transition from rupture to formation of black spots is described, as the surfactant concentration exceedsC bl. The remarkable fact in the latter case is the independence of the critical thickness of the final state, be it rupture or formation of first or second black films. On the basis of the experimental data the assumption is put forward that the critical thickness of rupture or the critical thickness of formation of black spots is substantially affected by macroscopic non-uniformities in the film thickness. Thus the conclusion is reached that the critical thickness of an ideally plane parallel film which is the object of the theory, must be obtained by extrapolation of the measured value toward extremely small radii.

The action of α-amanitin in vivo on the synthesis and maturation of mouse liver ribonucleic acids

Abstract: α-Amanitin acts in vitro and in vivo as a selective inhibitor of nucleoplasmic RNA polymerases. Treatment of mice with low doses of α-amanitin causes the following changes in the synthesis, maturation and nucleocytoplasmic transfer of liver RNA species. 1. The synthesis of the nuclear precursor of mRNA is strongly inhibited and all electrophoretic components are randomly affected. The labelling of cytoplasmic mRNA is blocked. These effects may be correlated with the rapid and lasting inhibition of nucleoplasmic RNA polymerase. 2. The synthesis and maturation of the nuclear precursor of rRNA is inhibited within 30min. (a) The initial effect is a strong (about 80%) inhibition of the early steps of 45S precursor rRNA maturation. (b) The synthesis of 45S precursor rRNA is also inhibited and the effect increases from about 30% at 30min to more than 70% at 150min. (c) The labelling of nuclear and cytoplasmic 28S and 18S rRNA is almost completely blocked. The labelling of nuclear 5S rRNA is inhibited by about 50%, but that of cytoplasmic 5S rRNA is blocked. (d) The action of α-amanitin on the synthesis of precursor rRNA cannot be correlated with the slight gradual decrease of nucleolar RNA polymerase activity (only 10–20% inhibition at 150min). (e) The inhibition of precursor rRNA maturation and synthesis precedes the ultrastructural lesions of the nucleolus detected by standard electron microscopy. 3. The synthesis of nuclear 4.6S precursor of tRNA is not affected by α-amanitin. However, the labelling of nuclear and cytoplasmic tRNA is decreased by about 50%, which indicates an inhibition of precursor tRNA maturation. The results of this study suggest that the synthesis and maturation of the precursor of rRNA and the maturation of the precursor of tRNA are under the control of nucleoplasmic gene products. The regulator molecules may be either RNA or proteins with exceedingly fast turnover.

Synthesis and maturation of ribosomal ribonucleic acids in isolated HeLa cell nuclei. A tracer study on the topology of the 45S precursor of ribosomal ribonucleic acids.

Abstract: The synthesis and processing of RNA by isolated HeLa cell nuclei was studied at low ionic strength in the presence of α-amanitin. The RNA polymerase reaction, with endogenous template and enzyme, rapidly reaches a plateau dependent on the amount of nuclei. Evidence is presented that incorporation of [3H]UMP proceeds only in growing RNA chains, whereas initiation of new RNA chains is arrested. The product formed contains all the main components of the 45S pre-rRNA (precursor of rRNA) maturation pathway (45S, 32S and 20S pre-rRNA; 28S and 18S rRNA). Most of the labelled material is in the mature rRNA components and their immediate precursors, even at very short times of incubation (2min). Small, but definite, 5S and 4S RNA peaks are also observed. At shorter incubation times a substantial amount of [3H]UMP is incorporated into RNA molecules in the 24S and 10–16S zones. This RNA material is considered to represent the non-conserved segments of 45S pre-rRNA in the process of nucleolytic degradation. A model for the tracer study of the topology of 45S pre-rRNA, on arrest of rRNA initiation, is discussed. The experimental evidence obtained supports the following structure of 45S pre-rRNA: 5′-end–28S rRNA unit–18S rRNA unit–nonconserved segment–3′-end.

On the mechanism of formation of amorphous condensates from the vapour phase (I). General theory

Abstract: The thermodynamic and kinetic conditions for the vapour condensation of non-crystalline thin films are formulated within the framework of the classical nucleation theory. The conditions for the predominant formation of crystalline or amorphous thin films are given and the acting supersaturation and the activity of the substrate are taken into account. Generalized criteria for the formation of amorphous (liquid or glassy) thin films are given and a temperature diagram is established which enables the concrete determination of the limits within which amorphous condensates are to be expected. It is thus possible to give the theoretical background of a technically and experimentally interesting process; the applications of the obtained results to the existing experimental data are also given in some detail. Further problems of the vapour condensation and stability of amorphous thin films, especially some effects of secondary crystallization will be discussed in more detail in Parts II and III of this series of papers, which report on experimental investigations of the formation of amorphous condensates.

Ultrastructural alterations of rat brain microglial cells and pericytes after chronic lead poisoning

Abstract: Wistar rats of both sexes and the same litter were exposed to chronic lead intoxication from birth untill sacrifice 9 months later. Lead was administered as 0.4% solution of lead nitrate in drinking water. Samples from the parietal brain cortex were examined electron microscopically following intracardiac perfusion with paraformaldehyde-glutaraldehyde solution. Similar changes were observed in the microglial cells and the vascular pericytes whereas all the other tissue elements appeared intact. Both cell types hypertrophied, the microglia assumed characteristic spindle or rod shape, the cell organelles increased, the microglial endoplasmic reticulum widened strongly and a large number of lipid inclusions appeared. The latter consisted of large lipid droplets of varying size and shape, containing multiple zones of low density, and a dense component with a coarse granular structure. The similarity in the response of both cell entities to the lesion as well as some probable functions of microglial cells and their relationship to vascular pericytes are discussed.

The Catalytic Oxidation of Hydrogen on Nickel Oxides

Abstract: The catalytic oxidation of hydrogen on highly-dispersed and sintered nickel oxides has been studied by a static method and the existence of two different kinetic rcgions established. Between 0 and 100°C the initial catalytic activity was not ionary and a strong poisoning effect of the reaction product was observed at all temperatnres up to 250°C. The activation energy of the reaction based on the initial reaction rates on freshly- outgassed oxide surfaces had a low value of 1–2 kcal. mole−1 with both preparations. Between 250 and 350°C stationary catalytic activity was observed and the activation energy of the reaction was significantly higher, 12–14 kcal . mole−1. The change of the activation energy is discussed in terms of the participation in the reaction of oxygen species in the catalyst surface layer which have different reactivities in the two temperature regions. A close analogy is noted between the carbon monoxide and hydrogen oxidation reactions on nickel oxide and a compensation effect is illustrated for a series of oxidation reactions on the oxide. Katalytische Oxydation von Wasserstoff an Nickeloxiden Die katalytische Oxydation von Wasserstoff an hochdispersen sowie gesinterten Nickeloxiden wurde mit einer statischen Methode untersucht und das Vorhandensein zweier verschiedener kinetischer Bereiche festgestellt. Zwischen 0 und 100°C war die anfangliche katalytische Aktivitat nicht stationar, und eine stark vergiftende Wirkung des Reaktionsproduktes wurde bei allen Temperaturen bis zu 250°C beobachtet. Die Aktivierungsenergie der Reaktion, die auf den anfanglichen Reaktionsgeschwindigkeiten auf frisch entgasten Oxidoberflachen basiert, wies den kleinen Wert von 1–2 kcal . mol−1 bei beiden Praparaten auf. Im Temperaturbereich zwischen 250 und 350°C wurde eine stationar katalytische Aktivitat beobachtet, und die Aktivierungsenergie der Reaktion lag bedeutend hoher bei 12–15 kcal . mol−1. Der Wechsel in der Aktivierungsenergie wird mit der Wirkung von unterschiedlichen Sauerstoffarten der Katalysatoroberflache, die in beiden Temperaturbereichen verschiedene Aktivitaten bei der Reaktion aufweisen, diskutiert. Eine Analogie zwischen der Kohlen- oxid- nnd Wasserstoffoxydation an Nickeloxid wird festgestellt sowie ein Kompensationseffekt fur eine Reihe von Oxydationsreaktionen gezeigt.

Smooth muscle cell regeneration in repair of gastric anastomosis in the dog.

Abstract: Tissue repair process after experimental transection of the stomach in dogs has been studied histologically and electron microscopically. Initially the incision gap is observed to be filled with loose connective tissue, in which smooth muscle cells (sm.m.c.) soon appear. On the 6th day after the operation single sm.m.c. are found dispersed among the polymorphous cell elements of the granulation tissue, comprising the area of the anastomosis. With the advance of the restorative process, increased number of sm.m.c. invides the connective tissue, replacing the defect. On the 9th and 12th days, islets of sm.m.c., often cone-shaped, adjacent with their bases to the normal muscular layer are observed in the connective tissue and within 30 to 90 days, bundles of sm.m.c., alternating with connective tissue, overbridge the operative defect and connect both edges of the transected gastric wall. The participation of the sm.m.c. in the process of repair of the gastric wall appears to be result of mitotic division of sm.m.c. from the transected muscular gastric layer and of migration of the new sm.m.c. into the focus of injury. Contrary to most concepts, that sm.m.c., as highly differentiated postmitotic cells, are restricted in their division capacity, the results of the present study indicate, that sm.m.c. in the mature organism of mammals preserve their ability for regeneration proper.

Dynamic NMR: A single operational parameter approach for evaluation of lifetime in an uncoupled, equally populated AB system

Abstract: Two simple formulae involving only one variable are presented for the simple and rapid estimation of the mean lifetime (τ) of exchange in uncoupled, paripartite AB systems. These formulae are shown to provide τ values comparable to those obtained from total line-shape analysis.

Latent Image Distribution in Evaporated Layers of Silver Bromide

Abstract: The distribution of development specks throughout the depth of evaporated layers of AgBr has been followed by means of graded dissolution. Nonsensitized layers were shown to build on exposure only internal latent image, with slightly higher concentration in the subsurface region. Negative imaging layers—gold-iridium sensitized, as well as direct positive layers—prefogged by vacuum deposition of silver specks have also been studied.Gold-iridium sensitization creates a reasonable photoresponse of the surface and increases to some extent the sensitivity of the subsurface region, leaving the volume practically unchanged. It is argued that one role of the sensitizer is to bind irreversibly the bromine equivalent created on exposure.The silver specks created by vacuum deposition are quite unstable and show a marked tendency to diffuse in the interior of the layers, probably along the interface boundaries. The photographic response in the interior of these layers depends on the intensity of exposure.